Gas scrubber

ABSTRACT

Apparatus for removing particulate material from a gas stream traveling through a duct comprising a pair of members each provided with a corresponding plurality of passages and arranged to cooperatively define a corresponding plurality of venturi passages extending generally parallel to the direction of flow of the gas stream and spaced around the axis of the duct. The members are relatively movable for varying the size of the venturi throats and for completely closing the passages when desired. Opposed sidewalls shaped to cooperatively define each venturi passage are carried by corresponding ones of the movable members whereby the configuration of the venturi passages is maintained throughout the entire range of adjustment. Liquid is supplied to the upstream member and the venturi inlets defined thereby are of a configuration effecting a substantially uniform distribution of the liquid to the passages.

Sawyer 1111 3,746,322 1 1 July 17, 1973 GAS SCRUBBER Inventor: Alan B.Sawyer, North Tonawanda,

[52] US. Cl 261/53, 261/109, 261/112,

261/113, 261/D1G. 54 int. Cl. B011 3/04 Field of Search 261/53, 62,108-113,

261/DIG. 54

References Cited UNITED STATES PATENTS 3/1916 Ferguson 261/114 VT 5/1916Ferguson 261/62 6/1934 Ketterer 26l/D1G. 54

2/1953 Olney 261/114 JP 7/1971 Katyal 261/11 B A l 60 A 1 PrimaryExaminer-Tim R. Miles Assistant Examiner-Steven H. Markowitv.Att0rney-Christel & Bean [57] ABSTRACT Apparatus for removingparticulate material from a gas stream traveling through a ductcomprising a pair of members each provided with a correspondingplurality of passages and arranged to cooperatively define acorresponding pluralityof venturi passages extending generally parallelto the direction of flow of the gas stream and spaced around the axis ofthe duct. The members are relatively movable for varying the size of theventuri throats and for completely closing the passages when desired.Opposed sidewalls shaped to cooperatively define each venturi passageare carried by corresponding ones of the movable members whereby theconfiguration of the venturi passages is maintained throughout theentire range of adjustment. Liquid is supplied to the upstream memberand the venturi inlets defined thereby are of a configuration effectinga sub stantially uniform distribution of the liquid to the passages.

9 Claims, 5 Drawing Figures PATENIED SHEET 1 0F 2 ens scnunnsnBACKGROUND OF THE INVENTION This invention relates to the gas scrubberart, and more particularly, to a new and improved wet collectoradvantageously including various desirable attributes of both theorifice and venturi types.

Wet collectors of the orifice type are well known and offer theadvantages of relative simplicity and low cost. However, thesecapabilities are limited, making them unsuitable for many installations.Wet collectors of the venturi type are capable of higher collectionefficiencies, the collection efficiency increasing with higher pressuredrops, and some venturi type collectors are designed with adjustablethroats to allow a range of pressure drops for a given gas volume.Further adjustment and regulation of the pressure drop to achieveeffective agglomeration has been accomplished by the provision ofmovable dampers in conventional venturi type scrubbers, but only withthe expenditure of con siderable energy. Also, such venturi scrubbersare relatively large and expensive.

A wet collector constructed to advantageously include various desirableattributes of both the orifice and venturi types is taught by Selway inUS. Pat. application Ser. No. 102,817 filed Dec. 30, 1970 and assignedto the assignee of the present invention. Briefly, the wet collector ofthat application utilizes first and second relatively movable membershaving corresponding passages which cooperatively define venturipassages in a manner permitting variation in the size of the venturithroats with a relatively small expenditure of energy. The members areidentical in construction and are relatively movable between a full openposition and a full closed position. Liquid is supplied at the peripheryof a surface of one of the members and rises to the level of one of theventuri passage inlet openings, the inlet openings being arranged so asto create a pressure differential which distributes the liquid uniformlyinto the openings.

In the operation of the wet collector of the foregoing application, theconfiguration of the venturi passage defining walls is altered slightlyas the members are moved between full open and full closed positions.While this usually will be of little or no concern under some operatingconditions, it is believed desirable to provide such a wet collectorhaving the additional capability of maintaining the same configurationin the venturi passage defining walls as the members are moved to varythe passage size.

SUMMARY OF THE INVENTION It is therefore an object of this invention tocombine the various desirable features of the orifice and venturi It isa further object of this invention to provide such a wet collectorwherein the pressure drop can be varied to provide effectiveagglomeration for various particle sizes and wherein a particularpressure drop can he maintained despite varying flow conditions, allwith a relatively minimal expenditure of energy.

It is a further object of this invention to provide such a wet collectorwherein the pressure drop can be varied over a significantly large rangeand can accommodate a wide range of gas flow conditions.

It is a further object of this invention to provide such.

passages in a manner permitting variation in the size of i the venturithroats with a relatively small expenditure of energy. Each of theventuri passages is defined, in part, by first and second walls carriedby said first and second movable members, respectively. Each of thewalls has a configuration for cooperatively defining, one with theother, the corresponding venturi passage, and the configuration isexactly maintained when the members are moved to vary the size of theventuri throats. Liquid is supplied at the periphery of a surface of oneof the members and rises to the level of the venturi passage inletopenings, the inlet openings being of a shape creating a pressuredifferential which distributes the liquid uniformly into the openings.

The foregoing and additional advantages and characterizing features ofthe present invention will become clearly apparent from the ensuingdetailed description together with the included drawings wherein:

BRIEF DESCRIPTION OF THE DRAWING FIGURES FIG. 1 is a fragmentaryelevational view, partly schematic, showing the apparatus of the presentinvention as it would appear in use;

FIG. 2 is a horizontal cross sectional view on an enlarged scale takenabout on line 2-2 of FIG. 1;

FIG. 3 is a developed vertical sectional view on a further enlargedscale taken about on line 3 3 of FIG. 2 showing the apparatus in a fullopened condition;

FIG. 4 is a developed vertical sectional view similar to that of FIG. 3but showing the apparatus in a full closed position; and

FIG. 5 is a developed vertical sectional view similar to that of FIG. 3but taken about on line 5 5 of FIG. 2 and on a somewhat reduced scale.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT FIG. 1 illustratesapparatus It) oi the present inven tion arranged to remove particulatematerial from a gas stream traveling through a duct II. The direction offlow of the gas stream is indicated by the arrow 12. The gas can be theeffluent from a blast furnace, in which case the upstream end of duct IIis suitably connected to the furnace exhaust, but it will be appreciatedthat apparatus of the present invention has the capability of scrubbinggas obtained from any of a wide variety of sources.

In the illustrated embodiment apparatus 10 comprises a first member 14and a second member I6 dcfining a plurality of venturi passages spacedaround the axis of duct 11 and disposed generally parallel to thedirection of flow of the gas stream as will be described in detail. Theapparatus 10 further comprises means for supplying liquid to the inletsof the passages, in particular to member 14, in the form of a supplyconduit indicated schematically at 18 connected to a liquid source (notshown), a water control device 20, and a liquid distributing conduit 22in communication with member 14 and connected through a conduit 24 tocontrol device 20. In a presently preferred form, member 14 includes agenerally horizontal planar surface, and liquid is supplied to thatsurface at or near the periphery thereof by conduit 22 whereupon theliquid level rises to the passage inlets.

The apparatus further comprises means operatively connected to at leastone of the first and second members 14 and 16, respectively, for causingrelative movement of members 14, 16 in generally parallel planes wherebythe throats of the venturi passages defined thereby can be varied incross sectional area. The

- means for causing relative movement can comprise an hydraulic cylinder26 suitably mounted or attached toa fixed reference surface relative toduct 11 and apparatus 10. The rod of cylinder 26 is connected to one endof an actuator arm or lever 28, the other end of which is attached orjoined to member 16, member 14 being stationary in the illustratedembodiment. The operation of cylinder 26 is controlled as a function ofthe pressure drop across the apparatus 10 in duct 11 as determined by afirst pressure sensing means 30 positioned in duct 11 upstream ofapparatus 10 and by a second pressure sensing means 32 positioned induct 1 1 downstream of apparatus 10. A pressure sensitive control deviceindicated schematically at 34 in FIG. 1 is connected through a line 35to sensor 30 and through a line 36 to sensor 32. Device 34, in turn,controls the flow of operating fluid to and from the rod and piston endsof cylinder 26 through lines 37 and 38. The detailed construction ofdevice 34 and of sensing means 30, 32 per se comprise no part of thepresent invention, and since they are readily commercially available invarioussuitable forms, all readily understood by those skilled in theart, a detailed description thereof is unnecessary.

Referring now to FIGS. 2-5,member 14 is formed to include a generallyplanar surface 40 and a peripheral edge portion in the form of askirt-like rim 41 disposed at about a right angle to the plane ofsurface 40. When member 14 is in the operative position shown in FIG. 1,surface 40 is disposed generally horizontally and rim 41 projectsupwardly in an upstream direction therefrom. In addition, rim 41 is ofan outer dimension and shape so as to provide a snug-fittingrelationship or engagement with the inner surface of duct ,1 1. Member14 is provided with a plurality of openings each designated 42 spacedaround the center thereof and extending through surface 40, which centercoincides with the axis of duct 11. In a similar manner, member 16 isformed to include a generally planar surface portion 45 and a peripheraledge portion in the form of a skirt-like rim (not shown) which isdisposed at about a right angle with respect to the plane of surface 45.This rim, like rim 4] of member 14, is ofa dimension and configurationso as to engage the inner wall surface of duct 11. In addition, member16 can be supported in duct 11 by a bearing band 70 FIG. I) suitablyattached to or formed on the inner surface of duct 11 and positionedwhereby the lower or depending edge of the rim of member 16 is seatedthereon. Member 16, like member 14 and duct 11, is circular and isprovided with a corresponding plurality of openings, each designated 49,spaced around the center thereof and extending through surface 45, whichcenter coincides with the axis of duct 11.

When assembled in duct 11, members 14 and 16 are rotatably connected atthe centers thereof by suitable means illustrated in the presentinstance as a bolt and nut 51. Members 14 and 16 cooperate to define aplurality of venturi passages spaced around the axis of duct 11, andeach passage extends generally in the direction of flow of the gasstream. The total number of venturi passages is equal to the number ofeither of the openings 42 or 49 in the members 14 or 16, respectively.Each venturi passage is defined in part by a first sidewall carried bymember 14 and extending from a corresponding opening 42. Sidewall 55comprises a middle portion 56 disposed generally perpendicular to theplanar portion 40 of member 14 and first and second end portions 57 and58, respectively, separated by middle portion 56. End portion 57 extendsupstream from surface 40 of member 14 and is disposed at a relativelyacute angle, for example, about 15 degrees, with respect to thedirection of flow of the gas stream. End portion 58 extends downstreamfrom surface 45 of member 16 and is disposed at a relatively acuteangle, for example 15 degrees, with respect to the direction of the gasstream. Each venturi passage is further defined by a second side wall 60carried by member l6 and including a middle portion 61 disposedgenerally perpendicular to planar portion 45 of member 16 and first andsecond end portions 62 and 63, respectively. End portion 62 extendsupstream from surface 40 of member 14 and is disposed at a relativelyacute angle, such as 15 degrees, with respect to the direction of flowof the gas stream. End portion 63 extends downstream from surface 45 ofmember 16 and is disposed at a relatively acute angle, such as 15degrees, with respect to the direction of flow of the gas stream. As aresult, walls 55 and 60 present a region defined by the opposed sidewallend portions 57 and 62 which converges in the direction of flow of thegas stream toward a throat region defined by sidewall middle portions 56and 61. The throat'region communicates with a region diverging from thethroat in a downstream direction and defined by the opposed sidewall endportions 58 and 63.

Each of the venturi passages is completed by opposed end walls whichengage corresponding ends of the sidewalls 55 and 60. In particular, endwalls 65 are carried by member 14, extend from openings 42, and contactthe radially innermost ends of sidewalls 55 and 60 relative to thecenter of member 14. End walls 66 are carried by member 16, extend fromopenings 49, and are generally coplanar with end walls 65. End walls 66as a result contact the radially innermost ends of the sidewall endportions 58 and 63. There also are provided end walls 67 carried bymember 14, extending from openings 42, and contacting the radiallyoutermost ends of sidewall portions 57 and 62. In a similar mannercorresponding and generally coplanar end walls 67' are carried by member16, extend from openings 49, and contact the radially outermost ends ofsidewall end portions 58 and 63. End walls 65, 66, 67 and 67' aredisposed in planes generally perpendicular to planar portions 40 and 45of members 14 and 16. The end walls preferably also are of a height orlength (in the direction of gas flow) greater than that of sidewalls 55and 60 whereby water flows only over the edges of sidewalls 55 and 60into the passage inlet openings. It will be appreciated that side walls60 have sliding contact with end walls 66 and 67'. Any spacingtherebetween is quite sufficient to provide clearance for relativemovement.

As a result, members 14 and 16 cooperate to define a plurality ofventuri passages spaced around the axis of duct ill and each extendinggenerally in the direction of flow of the gas stream. When member 14 islocated in duct 11 in the operative position of FIG. ll, each passagehas an inlet opening defined by the edges of end walls 65 and 67 and bythe edges of end portions 57 and 62 of sidewalls 55 and 60,respectively, which inlet opening is in communication with the dirty gasstream. Each of the passages, furthermore, is made to initially convergein the direction of flow of the gas stream or, in other words, in adirection from the inlet opening toward the plane of surface 40. Member16 preferably is identical to member 14, and when placed in the positionof FIG. 1 is inverted relative to member 14 whereby the correspondingpassage defining walls in member '16 diverge in a direction away frommember 14.

The number of venturi passages cooperatively defined by members 14 and16 will vary depending upon the size of duct 11. The end walls ofmembers 14 and 16 all are disposed generally parallel to the directionof gas flow thereby cooperatively defining relatively straight venturipassage end walls. Sidewalls 55 and 60 of members 14 and 16,respectively, include end portions 57 and 62 which converge in thedirection of gas flow and end portions 58 and 63 which diverge in thedirection of gas flow. As a result, there is provided a plurality ofventuri passages each having a restricted throat defined by the middleportions 56 and 61 of sidewalls 55 and 60, respectively. In the presentillustration member 16 is disposed so that planar portion 45 isgenerally horizontal, the rim or flange thereof contacts the innersurface of ductll, and the depending edge of the rimrests on a bearingband 70 or other suitable supporting arrangement whereby member 16 issupported for rotation about the axis of duct 11. Member 14 is thenplaced on member 16 and supported thereby in a manner defining theventuri passages as described hereinabove. Rim 41 of member 14 shouldcontact or engage the inner wall surface of duct It in a mannerproviding a fluidtight seal, and in some instances it may be necessaryor desirable to enhance the seal such as by a peripheral weld betweenflange 41 and duct 11. The foregoing arrangement for positioning members14 and 16 in duct 11 is illustrative, and other arrangements canbra-employed without departing from the spirit and scope of thisinvention.

Conduit 22 can be in the form of a manifold encircling duct II, withmultiple delivery conduits equally spaced thercaround, and it has beenfound that adequate distribution can be obtained with the number ofdelivery conduits equaling one-half the number of venturi passages, asclearly illustrated in FIG. 2.

The apparatus of the present invention operates in the following manner.Liquid, usually water, is delivered by conduits 22 to member 14 at theperiphery thereof and onto horizontal planar surface 40. The liquidfills the trough or reservoir defined by rimor flange 4i and the variousend walls 65, 6'7 and sidewalls 55 and 60 defining the venturi passageportions in member 14. The level of the liquid rises until it reachesthe height of sidewalls 55, whereupon the liquid enters the inletportions of the passages and flows generally in a direction indicated bythe dashed lines 7?. in FIG. 3. Sidewalls S5, 60 are of a heightslightly less than that of the end walls 65,67 and function as weirsover which the liquid flows. The liquid is atomized in the venturipassages by the moving gas stream, in a manner familiar to those skilledin the art, and the liquid droplets agglomerate with the solid particlesin the gas stream. The relatively clean gas together with theagglomerated liquid drops and particles leave the venturi passagesgenerally in a direction indicated by the arrows 74 in FIG. 3 forsubsequent separation of the liquid and agglomerated particles. Inparticular, separation of the agglomerated liquid drops and particlesfrom the gas stream can be accomplished by gravity, by effecting anabrupt change in the direction of flow, or by other methods andapparatus well known to those familiar with the art.

Substantially uniform distribution of liquid or water along the entirelength of end portions 57 and 62 of sidewalls 55 and 60, respectively,is accomplished by creation of a pressure differential in a radialdirection across each of the venturi passage inlet openings. Thepressure at or near the center of member M is made lower than thepressure at the periphery of member 14 by having each venturi passageinlet opening of a generally pie-shaped configuration as shown in FIG.2. In other words, each of the passage inlet openings has spaced apartside edges extending generally radially outwardly relative to the centerof member 14 which edges define therebetween an acute included angle.Therefore, the venturi passages are of progressively decreasingcross-sectional area, proceeding radially. inwardly from the outer endwalls 67, whereby the velocity of the gas passing through the passagesincreases progressively, again proceeding radially inwardly from endwalls 67. This results in a pressure gradient or progressivelydecreasing, radially inward form across member 14, producing a pressuredifferential causing liquid to flow radially inwardly across surface 40of member 14. In addition, it has been found that this configuration ofthe inlet openings adds more impetus to the fluid flow through theventuri throats.

To achieve the desired degree of agglomeration for optimum operation ofapparatus 1.0 it is necessary that the atomized liquid droplets be of asize substantially equal to that of the particles to be removed from thegas stream. The size of the liquid drops is, in turn, a function of thepressure drop across apparatus 10, Le. between the upstream side ofmember 14 and the downstream side of member l6. Accordingly, for a givensize of particle present in the gas stream, it may be necessary to varythe pressure drop to form liquid drops of the required size. Inaddition, if a characteristic of the flow should happen to change, suchas the volume, an adjustment most probably will be necessary to maintainthe desired pressure drop.

According to the present invention, the throat of each venturi passagecan be varied in size by a simple relative movement of members 1-4, 16,and this is accomplished with a relatively minimal energy expenditurearising from the action of essentially only shearing forces on membersl4, 16. In addition, this variation is provided by the apparatus of thepresent invention in a manner maintaining the same configuration of theventuri passages throughout the full range of adjustment. Referring nowto FIGS. 3 and 4, member 14 is moved by rotation in a plane normal tothe direction of gas flow to vary the throat size of the venturipassages cooperatively defined by members 14 and 16. For example, member16 can be rotated relative to member 14 between a fully opened positionwhere sidewall portions 56, 61 are spaced apart as shown in FIG. 3 and afully closed position where portions 56, 61 are in contact as shown inFIG. 4. During such movement, the portion of each venturi passageconfiguration defined by sidewalls 55 and 60 is maintained the samethroughout this range of adjustment by virtue of the fact that sidewall55 is carried by member 14 and sidewall 60, is carried by member 60 andthese sidewalls do not change shape. When member 16 is rotated, eachsidewall 60 is moved in its entirety toward and away from its associatedsidewall 55, such movement being in the region provided by openings 42in member 14 and openings 49 in member 16.

End walls 66 and 67' do move relative to their associated end walls 65and 66, but without interrupting or otherwise altering the end wallconfiguration in the space between the side walls, whereby only thewidth of the venturi passage end walls changes.

In the embodiment illustrated in FIG. 1, movement of member 16 iseffected by an hydraulic cylinder 26, and movement of the cylinder rodis transmitted through arm 28 to rotate the member 16. Movement ofmember 16 to vary the size of the venturi throats is performed as afunction of a pressure difference, under control of device 34 connectedto'cylinder 26 and to the pressure sensing means 30 and 32. Accordingly,member 16 is moved automatically to vary the cross sectional size of theventuri passage throats as a function of the pressure drop existingbetween the region upstream of member 14 and the region downstream-ofmember 16. Of course, the relative positioning of members l4 and 16 toselectively vary the throat size can be accomplished manually.

When a liquid or water seal is desired, this is accomplished by theapparatus of the present invention in a convenient, easy and economicalmanner. Member 16 simply is moved or rotated to the position of fullclosure indicated in FIG. 4. The supply of liquid to member 14 iscontinued, in a sufficient amount to effect a seal across the blockedthroats of the passages notwithstanding spaces which may exist betweenmembers 14, 16 because of manufacturing tolerances. it should be notedthat member 14 is in liquid-tight engagement with'the duct wall, as by awelded seam or other means Apparatus of the present invention issuitable for use with ducts of various diameters and requires onlyslight modification. For use in a relatively small duct, it has beenfound desirable to provide four venturi passages cooperatively definedby members 14 and 16. For relatively larger ducts the number of venturipassages is increased to as many as eight. it has been found thatincreasing the overall diameter of members l4, 16 requires a largernumber of passages each of a relatively smaller included angle.

The apparatus 10 of the present invention is of a relatively simpleconstruction, easily assembled and operated, and economical tomanufacture particularly when members 14 and 16 are of identicalconstruction. The apparatus has a high collection efficiency,approaching and sometimes exceeding that of a conventional venturiscrubber, while requiring a minimal expenditure of energy for varyingthe throat size of the venturi passages and requiring only a relativelysmall space. In addition, the configuration or shape of the venturipassages is maintained throughout the full range of variation in thethroat size, without interruption in either side or end walls. Effectiveliquid distribution is obtained by a radial pressure differential acrossthe upstream side of the scrubber. In addition, the apparatus permits awater seal to be provided in a convenient and easy manner for isolatingthe system in which it is included.

It is therefore apparent that the present invention accomplishes itsintended objects. While a single specific embodiment has been describedin detail, this has been done by way of illustration without thought oflimitation.

I claim:

1 Apparatus for removing particulate material from a gas streamtraveling through a duct, said apparatus comprising:

a. a first member positioned in said duct and pro vided with a pluralityof openings;

b. a second member positioned in said duct adjacent said first member onthe downstream side thereof, said second member being provided with acorresponding plurality of openings; I

c. a plurality of walls carried by said first and second members fordefining a plurality of venturi passages when the openings of said firstand second members are brought into alignment;

d. said walls including first and second sidewalls for each of saidventuri passages and carried by said first and second members,respectively, said sidewalls being shaped to cooperatively definetherebetween the corresponding venturi passage;

e. means for causing relative movement of said first and second membersthereby to vary the size of said venturi passages; and

f. means for delivering liquid to the inlets of said passages.

2. Apparatus according to claim 1 wherein each of said first and secondmembers comprises a generally planar portion from which said passagedefining walls extend and a peripheral edge portion adjacent the wall ofsaid duct.

3. Apparatus according to claim 2 wherein each of said sidewallsincludes first and second end portions separated by a middle portiondisposed generally perpendicular to the planar portions of thecorresponding first and second members, said first end portionsextending in a direction upstream from said first member and disposed ata relatively acute angle with respect to the direction of the gas streamso as to be converging toward said middle portions, and said second endportions extending in a direction downstream from said second member anddisposed at a relatively acute angle with respect to the direction ofthe gas stream so as to be diverging from said middle portions.

4. Apparatus according to claim 3 wherein one of each opposed pair ofsaid sidewalls defining each of said venturi passages is moved in itsentirety relative to the other thereof between an open position whereinsaid middle portions are spaced apart to a closed position wherein saidmiddle portions are in contact in response to relative movement of saidfirst and second members.

5. Apparatus according to claim I wherein said members are identical inconstruction, and are relatively movable between a full open and a fullclosed position.

6. Apparatus according to claim 2 wherein said first and second membersare disposed generally horizontally in said duct and wherein said liquiddelivering means comprises conduit means positioned to deliver liquid tosaid planar portion of said first member near said peripheral edgeportion thereof whereby liquid rises to a level corresponding to theheight of said sidewalls and then flows into said passages defined bysaid walls.

7. Apparatus according to claim 6 wherein said walls defining each ofsaid venturi passages are positioned relative to each other so as todefine passage inlet openings having a configuration which gives rise toa pressure differential between the center of said planar portion andsaid peripheral edge portion of said first memher for distributingliquid substantially uniformly to said passages.

8. Apparatus according to claim'7 wherein each of said passage inletopenings has spaced apart edges extending generally radially outwardlyrelative to the center of said planar portion which edges define anacute included angle.

9. Apparatus according to claim 1 further including: a. first pressuresensing means positioned in said duct upstream from said first memberrelative to the direction of flow of said gas stream; b. second pressuresensing means positioned in said duct downstream from said second memberrela tive to the direction of flow of said gas stream; and control meansconnected to said first and second sensing means and responsive to thepressure difference measured therebetween, said control meansbeingconnected in controlling relation to said means causing relativemovement between said first and second members whereby the throats ofsaid venturi passages are varied in response to changes in the pressuredrop across said apparatus.

2. Apparatus according to claim 1 wherein each of said first and secondmembers comprises a generally planar portion from which said passagedefining walls extend and a peripheral edge portion adjacent the wall ofsaid duct.
 3. Apparatus according to claim 2 wherein each of saidsidewalls includes first and second end portions separated by a middleportion disposed generally perpendicular to the planar portions of thecorresponding first and second members, said fIrst end portionsextending in a direction upstream from said first member and disposed ata relatively acute angle with respect to the direction of the gas streamso as to be converging toward said middle portions, and said second endportions extending in a direction downstream from said second member anddisposed at a relatively acute angle with respect to the direction ofthe gas stream so as to be diverging from said middle portions. 4.Apparatus according to claim 3 wherein one of each opposed pair of saidsidewalls defining each of said venturi passages is moved in itsentirety relative to the other thereof between an open position whereinsaid middle portions are spaced apart to a closed position wherein saidmiddle portions are in contact in response to relative movement of saidfirst and second members.
 5. Apparatus according to claim 1 wherein saidmembers are identical in construction, and are relatively movablebetween a full open and a full closed position.
 6. Apparatus accordingto claim 2 wherein said first and second members are disposed generallyhorizontally in said duct and wherein said liquid delivering meanscomprises conduit means positioned to deliver liquid to said planarportion of said first member near said peripheral edge portion thereofwhereby liquid rises to a level corresponding to the height of saidsidewalls and then flows into said passages defined by said walls. 7.Apparatus according to claim 6 wherein said walls defining each of saidventuri passages are positioned relative to each other so as to definepassage inlet openings having a configuration which gives rise to apressure differential between the center of said planar portion and saidperipheral edge portion of said first member for distributing liquidsubstantially uniformly to said passages.
 8. Apparatus according toclaim 7 wherein each of said passage inlet openings has spaced apartedges extending generally radially outwardly relative to the center ofsaid planar portion which edges define an acute included angle. 9.Apparatus according to claim 1 further including: a. first pressuresensing means positioned in said duct upstream from said first memberrelative to the direction of flow of said gas stream; b. second pressuresensing means positioned in said duct downstream from said second memberrelative to the direction of flow of said gas stream; and c. controlmeans connected to said first and second sensing means and responsive tothe pressure difference measured therebetween, said control means beingconnected in controlling relation to said means causing relativemovement between said first and second members whereby the throats ofsaid venturi passages are varied in response to changes in the pressuredrop across said apparatus.